1. Field of the Invention
The present invention relates to a method and apparatus for the production of liquid gas products, such as liquid natural gas, and, more particularly, relates to a reduction in the quantity of formed flash gas and in equipment requirements in a liquid gas manufacturing installation through the novel utilization of a bi-phase rotary separator for separating liquid and gas phases which, optionally, can also extract work from a liquid gas flow.
2. Discussion of the Prior Art
In liquid natural gas manufacturing facilities wherein liquid natural gas is conveyed from the manufacturing facility to a storage location subsequent to being flashed in a low pressure flash, a continuous stream of the liquid natural gas is conducted, as is well known, from a main exchanger of the liquid natural gas manufacturing facility through either a Joule-Thomson valve, or a reversely operating pump which removes work from the flow, and then to a low-pressure flash in which the effluent from the Joule-Thomson valve or a reversely operating pump is divided into a first flow consisting of liquid natural gas, and a second flow consisting of a natural gas vapor. The liquid natural gas is conducted through the intermediary of a suitable transfer pump to a storage facility, and the natural gas vapor is employed as fuel within the operating facility or plant.
Basically, the Joule-Thomson valve or reversely operating pump operate on the liquid natural gas stream flowing from the main exchanger of the liquid natural gas manufacturing facility at a very low, i.e., supercooled, temperature, thereby extensively reducing the temperature and pressure of the liquid flow. When a reversely operating pump is used within the facility, work is extracted and converted into mechanical or electrical energy through suitable shaft-coupled compressors, pumps or generators in order to power other installations or plant components. Thus, the energy state change through the Joule-Thomson valve, or the work recovered by the reversely operating pump, although relatively small in quantity, significantly enhances the production of liquid natural gas when flashed, while producing a lower volume of flash gas or natural gas vapor, thereby improving the economical operation of the manufacturing facility.
Although the utilization of Joule-Thomson valves and reversely operating pumps which extract work, such as centrifugal pumps or the like, result in an energy state change or the extraction of work from liquid streams under pressure, such as a cryogenic processing system for liquid natural gas which is conducted under high pressures and extremely low temperatures from the main exchanger of a liquid natural gas manufacturing facility, the energy state change or the work extracted has, generally, not been adequate to provide a degree of reduction in flash gas or natural gas vapor subsequent to flashing in a low pressure flash to a level of flash gas which meets the gas fuel requirements within the facility. Instead, an appreciable excess or waste of natural gases occurs, with a concommitant reduction in the production of processed liquid natural gas, so that the economic production potential of the liquid natural gas manufacturing facility is not fully realized.
In order to improve the economics of liquid natural gas production and to reduce the quantity of flash gas and resultingly increase the yield of liquid natural gas, an arrangement and method is disclosed in U.S. patent application Ser. No. 456,234, filed Jan. 7, 1983, for extracting work from a flow of liquid natural gas at extremely low temperatures with a hydraulic expander. The disclosure of this application is hereby incorporated herein by reference. The effluent from the hydraulic expander, when flashed in a low-pressure flash, produces a higher yield of liquid natural gas and, consequently, a lower proportion of flash gas, with an additional conservation of energy. The hydraulic expander is used in lieu of the conventional Joule-Thomson valve or reversely operating pump and the work extracted therefrom may be usefully employed in the facility to operate various power-driven components through suitable shaft-coupled compressors, pumps or generators. In a particularly advantageous arrangement, a Joule-Thomson valve is interposed in a flow conduit in parallel bypass relationship with the conduit incorporating the hydraulic expander and, in essence, is positioned intermediate a main exchanger of the liquid natural gas production facility and the low-pressure flash. The Joule-Thomson valve is closed during normal operation of the facility so as to render it inoperative and the entire flow of liquid natural gas passes through the hydraulic expander. The Joule-Thomson valve is rendered operative in an opened flow-through condition during periods when the hydraulic expander is shut down, or inoperative, to facilitate the continuous and uninterrupted operation of the liquid natural gas production facility, albeit at a somewhat lower degree of efficiency, without necessitating any shutdown of the system.
Although the just-described arrangement using a turbo-expander is advantageous in reducing the amount of produced vapor while extracting work from a flow of liquid natural gas, the vapor and liquid phase output from this device, in the form of effluent, must still pass to a conventional low pressure flash device for separating the liquid and vapor phases, with the liquid phase from the flash device then being pumped in conventional manner to a storage facility. The provision of a flash device and liquid pump naturally adds to the capital and maintenance costs of a liquid natural gas manufacturing installation.